1. Field of the Invention
The present invention relates to a method for manufacturing an electric charge transfer device, and more specifically to a method for manufacturing a charge transfer device of a two-phase drive, single-layer electrode structure having electric charge storage regions and electric charge barrier regions formed in a self alignment with conductive electrodes.
2. Description of Related Art
A buried channel charge transfer device of the two-phase drive, double-layer electrode structure is proposed by for example Choong-Ki Kim in "TWO-PHASE CHARGE LINEAR IMAGING DEVICES WITH SELF-ALIGNED IMPLANTED BARRIER", IEDM Tec. Digest, 1974, pp.55-58, the content of which is incorporated by reference in its entirety into this application.
Referring to FIGS. 1A to 1G, there are shown diagrammatic section views illustrating one example of the process, proposed by Kim, for manufacturing the buried channel charge transfer device of the two-phase drive, double-layer electrode structure.
First, as shown in FIG. 1A, in a principal surface of a P-type semiconductor substrate 501, there is formed an N-type semiconductor region 502, which is of the conductivity type opposite to that of the substrate 501, and a first insulator film 503 is formed on a surface of the N-type semiconductor region 502 by a heat oxidation.
Then, as shown in FIG. 1B, first conductive electrodes 509 composed of a polysilicon are formed on the first insulator film 503 by a well known technique.
Succeedingly, as shown in FIG. 1C, the first insulator film 503 is selective removed by using the first conductive electrodes 509 as a mask, and then, a second insulator film 512 is formed by a heat oxidation.
Thereafter, as shown in FIG. 1D, impurity of the conductivity type opposite to that of the N-type semiconductor region 502, for example, boron, is introduced into the N-type semiconductor region 502 between the first conductive electrodes 509, by an ion implantation, so that N.sup.- semiconductor regions 507 are formed in self alignment with the first conductive electrodes 509.
Succeedingly, as shown in FIG. 1E, second conductive electrodes 513 composed of a polysilicon are formed on the second insulator film 512 by a well known technique in such a manner that each of the second conductive electrodes 513 is positioned above a corresponding N.sup.- semiconductor region 507 to partially overlap an adjacent end of each of adjacent first conductive electrodes 509.
Then, as shown in FIG. 1F, an interlayer insulator film 510 is formed to over all the surface.
Furthermore, as shown in FIG. 1G, each of pair of adjacent first and second conductive electrodes 509 and 512 are interconnected to form an electrode pair by means of contact holes formed through the interlayer insulator film 510 and the second insulator film 512, and electrode pairs thus obtained are alternately connected to a pair of metal wiring conductors 511, respectively, which are driven with a pair of transfer drive pulses .PHI.H1 and .PHI.H2, respectively. Thus, the charge transfer device of the two-phase drive, double-layer electrode structure is obtained.
With a recent advanced micro-fabrication technology, it has become possible to form, in place of the above mentioned double-layer electrode structure, a charge transfer device of a single-layer electrode structure having an inter-electrode distance or spacing on the order of 0.2 .mu.m to 0.3 .mu.m, by etching a single layer electrode material.
The charge transfer device of the single-layer electrode structure is more advantageous over the double-layer electrode structure in that, since there is no overlapping between adjacent electrodes, an interlayer capacitance is small, and there is no problem of insulation between the electrodes. In addition, since it is not necessary to oxidize the electrode in order to form the interlayer insulator, it is possible to use, as an electrode material, a metal film or a silicide film in addition to the polysilicon, thereby to reduce the resistance of the electrode.
Referring to FIGS. 2A to 2G, there are shown diagrammatic section views illustrating one example of the process for manufacturing a buried channel charge transfer device of the two-phase drive, single-layer electrode structure.
First, as shown in FIG. 2A, in a principal surface of a P-type semiconductor substrate 601, there is formed an N-type semiconductor region 602, which is of the conductivity type opposite to that of the substrate 601, and a first insulator film 603 is formed on a surface of the N-type semiconductor region 602 by a heat oxidation.
Then, as shown in FIG. 2B, by using as a mask a photoresist 605 patterned by a photolithography, impurity of the conductivity type opposite to that of the N-type semiconductor region 602, for example, boron, is introduced into the N-type semiconductor region 602 by an ion implantation, so that N.sup.- semiconductor regions 607 are selectively formed.
Succeedingly, the photoresist 605 is removed, and as shown in FIG. 2C, a polysilicon layer 608 is formed on the first insulator film 603 by a well known technique.
As shown in FIG. 2D, a patterned photoresist 614, which is used for a mask for patterning the polysilicon layer 608, is formed on the polysilicon layer 608 by a photolithography.
Then, the polysilicon layer 608 is etched using the patterned photoresist 614 as the mask, so that the polysilicon layer 608 is divided into a number of conductive electrodes 609, as shown in FIG. 2E.
Then, as shown in FIG. 2F, an interlayer insulator film 610 is formed to over all the surface.
Furthermore, as shown in FIG. 2G, the conductive electrodes 609 are alternately connected to a pair of metal wiring conductors 611, respectively, which are driven with a pair of transfer drive pulses .PHI.H1 and .PHI.H2, respectively. Thus, the charge transfer device of the two-phase drive, single-layer electrode structure is obtained.
In the above mentioned prior art charge transfer device of the two-phase drive, single-layer electrode structure, however, since the conductive electrodes 609 and the N.sup.- semiconductor regions 607 (electric charge barrier region of the transfer channel) are not formed in self alignment with each other, the conductive electrodes 609 and the N.sup.- semiconductor regions 607 are often deviated from a proper position, as shown in FIGS. 3A and 3B. If the conductive electrodes 609 and the N.sup.- semiconductor regions 607 are deviated as shown in an upper half of FIG. 3A, a projection "A" is generated in a potential profiles as shown in a lower half of FIG. 3A. If the conductive electrodes 609 and the N.sup.- semiconductor regions 607 are deviated as shown in an upper half of FIG. 3B, a concave "B" is generated in a potential profiles as shown in a lower half of FIG. 3B. The projection "A" and the concave "B" of the potential profiles become a hindrance of a smooth electric charge transfer.